const findNthDigit = (n) => {
  let digit = 1;
  let start = 1;
  let count = 9;
  while (n > count) {
    n -= count;
    digit += 1;
    start *= 10;
    count = 9 * start * digit;
  }
  // 确定所求数位所在的数字 
  // 确定所求数位在 num 的哪一数位
  const num = start + (n - 1) / digit + "";
  return num.charAt((n - 1) % digit) + "";
};
